1. Field of Invention
The instant invention relates to surgical implants and related systems for use with surgical implants, inclusive of orthopedic and dental implants for the purpose of enhanced osseo-integration of the implant into the surgical site to further post-operative efficacy of the implant-related procedure. The present invention is particularly of interest in use with metallic implants such as those of titanium, titanium alloy, titanium/aluminum/vanadium alloy, zirconium and tantalum and metallic implants coated with an osseo-simulative material such as hydroxyapatite.
2. Dental Implants
There exist many implants; each designed for a specific function. Most are made of titanium, an inert metal which has been proven to be effective at fusing with living bone, a process known as xe2x80x9cosseointegration.xe2x80x9d The cylindrical or screw type implant called xe2x80x9croot formxe2x80x9d is similar in shape to the root of a tooth with a surface area designed to promote good attachment to the bone. It is the most widely used design and generally placed where there is plentiful width and depth of jawbone. Where the jawbone is too narrow or short or immediate placement of root form implants the area may be enhanced with bone grafting to allow for their placement.
When the jawbone is to narrow and is a good candidate for bone grafting, a special narrow implant, called xe2x80x9c plate form,xe2x80x9d can be placed into the bone, i.e., in cases of advanced bone loss, such a subperiosteal implant may be prescribed. It rests on top of the bone but under the gums.
The actual implant procedure involves the surgical placement of an implant, a healing period (osseointegration), and implant restoration to replace the missing tooth or teeth. The treatment may be a cooperative effort between a surgical dentist who actually places the implant and a restorative dentist who designs, prescribes and inserts the final replacement teeth. Some dentists having advanced training provide both of these services.
Root form implants are the closest in shape and size to the natural tooth root. They are commonly used in wide, deep bone to provide a base for replacement of one, several, or a complete arch of teeth. After application of an anesthetic, the dentist will expose the area of the jawbone to be implanted and prepare the bone to accept the implant. The number of incisions and manner of bone preparation depends upon the number of implants (and teeth) to be replaced. The implant is carefully set into place and the gums are closed with several stitches. The healing period usually varies from three to six months. During this time osseointegration occurs. The bone grows in and around the implant creating a strong structural support. In fact, this bond can be even stronger than the original tooth""s. When healing is complete, the implant is uncovered and an extension or abutment is attached to it. At that time, the implant and abutment act as a solid unit ready to support the patient""s new tooth or teeth.
3. Orthopedic Implants
Owing to the rapid development of surgery, it is now possible to carry out operations to bones and joints which were recently inconceivable. For example, it is now possible to carry out surgical removal of cysts, foci of suppuration in bone, and several of malignant tumors from bones. This results in defects in the bone, which need to be filled since normal bone repair processes are no longer able to compensate them. Some defects of this type may have a volume of up to 600 cm to be filled.
For filling cavities of this type use is made of bone replacement materials in liquid, pasty or a solid form such as granules or implants for implantation. If the cavities which are to be filled are not too large, then the purpose of the bone replacement materials is to temporarily fill the cavities in the bone and to allow the body itself to compensate, in the course of time, for the defect with living bone material.
4. Bio-compatible Materials
Bone implants are frequently used in surgical procedures, which are implanted in the bones of the body of a recipient and permanently replace parts of the skeleton or roots of teeth. The outer layer of the bone implant, which comes into contact with the living substrate bone, is termed the bone-contact layer. At the present time, metals, such as, for example, special steels, noble metals, titanium, ceramic materials, such as, for example, alumina,. glass-ceramics, hydroxy-apatite ceramics and synthetic materials are used as bone implants and as bone-contact layers.
These substances are classified as biocompatible and bioactive according to the tissue compatibility. Biocompatible substances are tolerated by the body in the long term without rejection. Bioactive substances become rigidly incorporated like endogenous tissue, the tissue compatibility determined by the chemical composition, the crystalline structure, the surface structure and the mechanical properties.
The metals and some ceramic materials such as, for example, alumina ceramics, are biocompatible. Ensheathing by connective tissue always takes place in the body. This connective tissue layer allows the implant to be held relatively rigidly, but does not allow frictional connection to the mineral framework of the substrate bone.
Because of the absence of primary integration into the substrate bone, a biocompatible implant of this type can be exposed to only slight mechanical stress since otherwise it is held poorly, which is associated with pain and, finally, the loss of the implant. This is found, for example, with hip joint prostheses which are always subject to great stress and for which more than one quarter of the operations are carried out are because of loosening of an implant which had previously been inserted.
Thus, additional undercutting such as, for example, a screw thread is necessary for permanent mechanical anchoring of biocompatible implants in bone. With all metallic implants it is still an unanswered question of whether they release toxic metal ions into the surroundings and thus may have adverse effects in the long term. Even when bone cement is used, despite the initially better mechanical connection to the substrate bone, the loosening above described takes place, although with some delay.
In about the last twenty (20) years, implant techniques that employ many artificial hard tissue materials have been used surgeons. Among these materials, bioglass and bioceramics, such as hydroxyapatite and beta-tricalcium phosphate, have excellent biocompatibility. Most of the bioglass and bioceramics for medical applications are prepared either in granule or block form. The granule form has mobility problems and relatively poor manipulation characteristics, while the block form is quite brittle and difficult to shape. Many other techniques have been attempted to solve the above-noted problems. Various of these techniques have employed other materials such as: plaster of paris (calcium sulfate), CS hemihydrate, collagen, different types of calcium phosphate grout or cement, polylactates and polyacrylate cement compositions. None of these have been completely acceptable.
5. Desired Parameters
The surgeon is often interested in implant techniques that employ materials that can be shaped and hardened in situ. Ideally, an effective implant technique should employ a surgical cement or binder system for hard tissue applications, having the following characteristics: good biocompatibility, a suitable resorption rate, moldable at the surgical site, and controllable setting time and characteristics.
Most currently techniques employing available surgical cements and binder composition system have disadvantages. For example, collagen-hydroxyapatite and polylactate-hydroxyapatite composites can only be made as premolded shapes and cannot be molded at the surgical site.
Generic plaster of paris, which is derived from gypsum, has reasonable setting characteristics but its resorption rate is too fast. Polyacrylate cement is non-resorbable. Polyacrylic acid-calcium phosphate cement is not resorbable and the setting cement is too acidic. Most of the calcium phosphate grouts or cement composition are prepared by the reaction of calcium phosphate ceramics with an acidic component. See, for example, Bajpai, U.S. Pat. No. 4,663,295. In general, these cement compositions are disadvantageously acidic in nature and take too long to reach a neutral pH. The calcium phosphate grouts or cement compositions either lack satisfactory mechanical strength or are resorbed too slowly. Moreover, most prior art calcium phosphate cement compositions developed require the use of hydroxyapatite or tricalcium phosphate as the cementing ceramic and phosphoric acid, a bifunctional organic acid or other polyfunctional organic acids as a setting reagent. These cement compositions are also very acidic in nature and take too long to reach a neutral pH. Also, after implantation, these cement compositions may cause irritation and inflammatory reactions. Thus, surgical techniques employing these materials have not proven to be totally satisfactory.
Biocompatibility has also been a limiting factor in successful application of implant cement compositions The most successful artificial implant materials to achieve the excellent biocompatibility have been hydroxyapatite, bioglass, and other calcium phosphate ceramics. Bioglass is a bioactive glass material whose major components are CaO, SiO2 and P2O. Minor components may be Na2O, MgO, Al23, B2O3 and CaF2. A bioactive glass can form a surface layer of hydroxyapatite when soaked in an aqueous environment. Hydroxyapatite with beta-tricalcium phosphate ceramics and calcium phosphate containing glass have been extensively studied. Clinical studies confirmed that most of the calcium phosphate ceramics such as hydroxyapatite, tricalcium phosphate, tetra calcium phosphate, and dicalcium phosphate have excellent biocompatibility and are well accepted by both hard tissue and soft tissue. Experimental results also indicate that dense hydroxyapatite is nonbioresorbable while other porous calcium phosphate ceramics are bioresorbable. However, surgical techniques employing these materials have not proven satisfactory
6. Calcium Sulfate (xe2x80x9cCSxe2x80x9d)
CS was first utilized as a filler for bone defects by Dreesmann in 1892 and, in the first scientific studies undertaken in the 1950s, Peltier demonstrated that CS can be an effective resorbable material for filling bone defects and retaining bone grafts. Peltier noted that CS resorbs rapidly, causes no inflammation, evokes minimal foreign body response, results in normal regeneration of bone, substantial resorption of the CS material, and causes no measurable rise in serum calcium levels. Composites of CS and hydroxylapatite have been shown to induce bone growth (osteoconductivity).
Other related prior art, known to the within inventors, is reflected in U.S. Pat. No. 4,381,947 to Pellico (1983) entitled Settable Alginate Compositions which include Calcium Sulfate; U.S. Pat. No. 5,147,403 (1992) to Gitelis, entitled Prosthesis Implantation Method, which teaches the use of a suspension of CS hemihydrate on a receiving surface of a host bone such that the seating of an orthopedic prosthesis thereon will result in an enhancement of fibroblast growth factor (FGF) at the implant interface.
Later patents which reflect the utility of CS and compounds thereof for use as both a delivery and bone ingrowth medium include U.S. Pat. Nos. 5,366,507 (1994) and 5,569,308 (1996), both to Sottosanti, and both entitled Method for Use in Bone Regeneration; and U.S. Pat. No. 5,807,567 (1998) to Randolph, et al, entitled CS Controlled Release Matrix for use in Delivery of Antibiotics. U.S. Pat. No. 4,619655 (1986) to Hanker, et al teaches the use of CS as a resorbable scaffold with bone implants. U.S. Pat. No. 5,521,265 (1993) to Liu teaches surgical cements which include calcium compounds.
It has therefore been known that CS hemihydrate possesses excellent bio-compatibility combined with rapid dissolution which allows use as a carrier for delivery of soluble agents. See Rosenblum, et al, Material Research Symposium Proceedings, Materials Research Society, Volume 252 (1992), and Ricci, et al, Trans Society of Materials 15:49 (1992); As such, it is known that permanent and resorbable coatings of CS hemihydrate will stimulate in vitro bone ingrowth in an implantable chamber. See also Ricci, et al, Permanent and Resorbable Coatings for Bone Ingrowth into Porous Beaded Surfaces, 23 Annual Meeting of Society for Biomaterials (1997). CS therefore has many options for use in bone repair. For example, it may be used alone as a defect filler, as a binder for retention of other materials at a defect site, or as a resorbable barrier to reduce ingrowth from soft tissue near an osseotomy site. Further, the ability of CS to be easily absorbed makes it a suitable delivery vehicle for such materials as growth factors, osteogenic factors and antibiotics. It is currently in use to stimulate bone regeneration, in sinus lift procedures, in oral surgery applications such as filling of cysts or defects left by the removal of impacted wisdom teeth, in endodontic procedures, and in treating periodontal defects. Notwithstanding the above, practicable CS based systems are lacking in many other surgical applications. As such, a number of issues and factors have impeded the development of CS based systems in bone implant and repair procedures. One such problem has been the ingrowth of unwanted tissue into an implant or graft site from adjacent soft tissue to the implant site. This problem has given rise to the concept of a xe2x80x9cbarrier layerxe2x80x9d to protect the graft material or implant from soft tissue disruption. The present invention thereby has, as one objective, the provision of improved barrier methods associated with the use CS systems used in bone tissue regeneration.
A further problem associated with the prior art has been that of synchronizing the rate of in situ bio-resorption of the CS compound/system with the healing process of the adjacent osseous tissue. As above noted, a persistent problem has been that a given CS system will bio-resorb or dissolve too rapidly within the bone thereby outpacing the formation of new bone in human tissue. When this occurs, the effective period of the CS system is significantly diminished. More particularly, when CS is used as a cement to fill a bone void, fracture, or other defect, this material dissolves at a rapid rate, i.e., approximately one millimeter per week from the outside of the defect towards the center thereof. Research of the inventors has shown that this material causes precipitation of calcium phosphate (CP) deposits as it is resorbed by the bone. These precipitates, it has been shown, stimulate and direct the formation of new bone. It is however important for purposes of optimal result that CS, CP, or any other bone repair material stay at the surgical site for a considerable period in order to inhibit soft tissue filling of the defect and to induce proper bone repair. As such, the principal concern and difficulty expressed by practitioners (such as orthopedic and maxiofacial surgeons) using such materials is that they bio-resorb or dissolve too rapidly within the bone and, thereby, outpace the formation of new bone in human patients.
The invention therefore relates to preferable CS combinations or matrices for use with an implant for the repair, augmentation, and other treatment of bone. Such combinations, as set forth below, possesses significant advantages over CS cements and pellets which are currently in clinical use. More particularly, current CS materials are resorbed by human bone within two to seven weeks, depending upon the CS form and the particular surgical site, however, cannot be retained at the site for longer periods. As noted, such material is resorbed faster than it can be replaced by new bone thereby reducing its value to both patient and practitioner. The invention, as such, embodies a CS system particularly formulated and physically configured to be bio-resorbed at controlled rates, thereby substantially matching its rate of resorption to that of the rate of bone repair at specific surgical sites in various applications.
The instant invention relates to a surgical implant system comprising an implant body and an osseo-stimulative surface applied to or used with said implant body, said surface including a calcium sulfate (CS) compound which is a member selected from the group consisting of CS dihydrate, CS hemihydrate, anhydrous CS and mixtures thereof. The performance and rate of resorption of said osseostimulative surface may be improved or modified through the use of a stabilizing component, a viscosity modifier, a pH modifier, or a cell growth inductive microgeometry. The system is also definable in terms of an in situ system of bone augmentation in which a bio-resorbable CS matrix from a kit is disposed about the surgical implant positioned within an osseotomy site.
The invention, in kit form, will typically include a standalone quantity of a CS compound for use with the surgical implant at the osseotomy site.
Therefore, in view of the above, it is a primary object of the invention to provide an improved article and system for the implantation of prostheses, such as dental implants, and for in situ bone repair.
It is another object to provide an improved means of enhancing osseointegration between a surgical implant or graft and adjacent osseous tissue.
It is a yet further object of the invention to provide a system of the above type which includes an improved barrier layer for the protection of the implant or graft site from disruption by ingrowth of soft tissue, which barrier may resorb after the osseous healing process has been completed.
The above and yet other objects and advantages will become apparent from the hereinafter set forth Brief Description of the Drawings, Detailed Description of the Invention and Claims appended herewith.